In recent years, with the spread of a high-performance portable wireless station, such as a notebook computer or a smartphone, wireless LANs in compliance with the IEEE 802.11 standard have come into wide use not only in companies and public spaces, but also in ordinary houses. As the wireless LANs in compliance with the IEEE 802.11 standard, there are a wireless LAN in compliance with the standard IEEE 802.11b/g/n that uses a 2.4 GHz band, and a wireless LAN in compliance with the standard IEEE 802.11a/n/ac that uses a 5 GHz band.
At this point, in a wireless LAN high density environment where the number of basic service sets (BBSs) is greater than the number of channels that are available at the same time in the same place, a plurality of BSSs use the same channel. In that case, due to interference among the BSSs that use the same channel, throughput of the BSS and throughput of an entire system are decreased. For this reason, in the wireless LAN, autonomous distributed access control called CSMA/CA that performs transmission of data only when the channel is unoccupied.
However, although the CSMA/CA protocol is performed, in the wireless LAN high density environment where many wireless LAN networks use the same channel, because the frequency with which the channel is busy due to the carrier sense is increased, a transmission opportunity (an opportunity to obtain the channel access right) is reduced, and throughput is reduced.
When the wireless station performs the carrier sense, a clear channel assessment (CCA) threshold, based on which it is determined whether a channel is idle or busy, using a received signal strength indication (RSSI) of a receive signal, is set. With dynamic sensitivity control (DSC) that dynamically controls the CCA threshold, a technique in which a state where more defer transmission than is necessary take place is removed and thus the transmission opportunity is increased has been studied (Non-Patent Documents 1, 2, and 3). A control example of the CCA threshold will be described below, taking, for example, downlink communication from an AP to an STA in which the access point and the station are defined as the AP and the STA, respectively.
FIG. 14 illustrates a control example of the CCA threshold in the related art.
In FIG. 14, an AP 1 and an STA 1, an AP 2 and an STA 2, and an AP 3 make up different BSSs, respectively. The AP 1 possibly selects a default CCA threshold of −82 dBm and a CCA threshold of −62 dBm for a Non-WLAN frame. When a signal that is detected by carrier sense in accordance with the default CCA threshold is, for example, a signal other than a signal for a wireless LAN, the AP 1 performs control in such a manner that changing to a CCA threshold for a Non-WLAN frame takes place and thus a carrier sense range is narrowed down, thereby increasing a transmission opportunity. An example is described here in which the AP 2 is located within a carrier sense range in accordance with two CCA thresholds of the AP 1 and in which the AP 3 is located out of a carrier sense range in accordance with the default CCA threshold.
The AP 1 starts to perform carrier sense at the default CCA threshold of −82 dBm (S100), and, if a RSSI of a receive signal is at a receiver sensitivity of −82 dBm or less (Yes in S101), detects a preamble of the receive signal (S102). If the preamble is normally detected (Yes in S103), a channel state where setting to the default CCA threshold at which the detection of the preamble is successful takes place is determined as is (S104). At this point, if a channel is busy (Yes in S105), demodulation of the receive signal is continued (S106), and if the channel is idle (No in S105), the demodulation of the receive signal is stopped (S107). On the other hand, if the preamble is not normally detected (No in S103), changing to the CCA threshold of −62 dBm for the Non-WLAN frame takes place and a transmission opportunity is caused to be increased by narrowing down the carrier sense range and determining the channel state (S108).
In the CCA threshold control described above, in the AP 1, even if the preamble can be normally detected when a signal of the AP 3 is at receiver sensitivity or above, the channel is determined to be idle by setting the CCA threshold to the default value, and the demodulation is stopped. Thus, the transmission opportunity can be caused to be increased. That is, the AP 1 and the AP 3 possibly perform simultaneous transmissions with mutual interference being decreased. On the other hand, regarding a signal of the AP 2, the channel is busy by causing the setting to the default CCA threshold to take place. The AP 1 continues the demodulation of the signal of the AP 2 and defers transmission. At this point, although the simultaneous transmissions from the AP 1 to the STA 1 and from the AP 2 to the STA 2 are possible, the detection of the signal of the AP 2 causes the AP 1 to defer transmission and to be in a state of a so-called exposed station. For example, regarding the AP 1, if it can be detected that a signal of another BSS is present and if the CCA threshold is controlled in a manner that causes the CCA threshold to be high and the corresponding carrier sense range is narrowed down, although the AP 2 is performing the transmission, the simultaneous transmission by the AP 1 is possible and system throughput is improved.
However, if the STA 1 is located near the AP 2 and an interference signal from the AP 2 is large in magnitude, although the control of the CCA threshold in the AP 1 causes the transmission opportunity of the AP1 to be increased, there is a concern that a receive failure will occur in the STA 1 due to the interference signal from the AP 2, thereby instead decreasing throughput.
Furthermore, in the AP 1, if a signal that is transmitted by the STA 1 under control is received, or if a beacon frame that is transmitted by the AP 2 is received, it can be said that the AP 1 may be kept from performing the transmission and perform the reception, rather than controlling the CCA threshold and increasing the transmission opportunity. That is, it is desirable that the AP 1 distinguishes between the signal from the STA 1 in the BSS that the AP 1 itself makes up, and the signal from the AP 2 in another BSS, and that AP 1 can select whether or not the CCA threshold is controlled and thus the simultaneous transmissions are set to be possible, depending on whether or not the demodulation of the signal from the AP 2 in the another BSS is also necessary.    Non-Patent Document 1: “Proposed Specification Framework for TGax”, Robert Stacey et al., doc.:IEEE 802.11-14/1453r2, 5 Nov. 2014    Non-Patent Document 2: The LAN/MAN Standards Committee”, IEEE Std 802.11TM 2012 IEEE standard for Information Technology—Telecommunications and information exchange between systems—Local and metropolitan area networks Specific requirements Part11:Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications.    Non-Patent Document 3: Graham Smith, “Dynamic Sensitivity Control Practical Usage”, doc.: IEEE 802.11-14/0779r2